Abstract
The Wolbachia are intracellular endosymbionts widely distributed among invertebrates. These primarily vertically transmitted α-proteobacteria have been intensively studied during the last decades because of their intriguing interactions with hosts, ranging from reproductive manipulations to mutualism. To optimize their vertical transmission from mother to offspring, the Wolbachia have developed fine-tuned strategies. However, the Wolbachia are not restricted to the female gonads and frequently exhibit wide intra-host distributions. This extensive colonization of somatic organs might be necessary for Wolbachia to develop their diverse extended phenotypes. From an endosymbiont's perspective, the within-host environment potentially presents different environmental constraints. Hence, the Wolbachia have to face different intracellular habitats, their host's immune system as well as other microorganisms co-occurring in the same host individual and sometimes even in the same cell. A means for the Wolbachia to protect themselves from these environmental constraints may be to live 'hidden' in vacuoles within host cells. In this review, we summarize the current knowledge regarding the extent of the Wolbachia pandemic and discuss the various environmental constraints these bacteria may have to face within their 'host ecosystem'. Finally, we identify new avenues for future research to better understand the complexity of Wolbachia's interactions with their intracellular environment.